Due to emission of greenhouse gas caused by the use of fossil fuels, global warming has resulted in changes in climate and global environment, thereby threatening the survival of all living things on earth including human beings. Accordingly, various researches and developments for reducing carbon dioxide are now in progress. As one of efforts, study on the way of capturing and biologically converting carbon dioxide is actively underway.
As a green plant which performs photosynthesis for biologically converting carbon dioxide, microalgae have been actively studied. In common with other green plants in photosynthetic process, microalgae, phytoplankton, use sun as an energy source and grow up with photosynthesis for biofixation of carbon dioxide.
The first reason for recognizing microalgae as a means of biofixation of carbon dioxide is very low amount of energy to be injected for capturing carbon dioxide, because solar energy may be the main energy source as the same with absorption of carbon dioxide. Thus, since there is less amount of generating carbon dioxide for operation of biofixation of carbon dioxide, removal efficiency is high in terms of profit balance of carbon dioxide.
Secondly, required size of site is small because of very high speed of fixation of carbon dioxide as compared to green plants. According to results from study conducted at Tokyo Electric Power Research Institute, it is revealed that the speed of fixation of carbon dioxide of microalgae is more than 8 times higher than that of macroalgae and more than 16 times higher than that of pine trees, the most common tree in Korea.
Besides, there is an advantage in that processes for separating and concentrating carbon dioxide are not required due to direct fixation of carbon dioxide from combustion gas. Moreover, microalgae, generated from carbon dioxide fixation, contain numerous useful materials, thereby being utilized for manufacture of expensive bioproducts.
These microalgae are cultured by using open or closed-ended photochemical reactors. Especially, as for closed-ended photochemical reactors, there is a gradual increase in frequency of use because closed-ended photochemical reactors can culture more highly concentrated microalgae than those by open photochemical reactors and high value materials can be produced from the cultured microalgae. As for microalgae cultivation using such closed-ended photochemical reactors, a plurality of photochemical reactors having a few drops of culture fluid in terms of easy-to-use sunlight has to be used. To do so, gas with a constant amount (especially, carbon dioxide) has to be stably supplied to such multiple photochemical reactors.
However, there has been problems that it is mostly difficult to take samples due to characteristics of closed-ended photochemical reactors; agitation and light use efficiencies are reduced because microalgae is precipitated while cultivation is underway; and it is hard to harvest microalgae from a plurality of photochemical reactors after completion of cultivation.